Injection nozzle-pump combinations have been proposed which use a pump piston of a diameter smaller than the diameter of a servo piston connected thereto, the servo piston being subjected to fluid pressure by a servo fluid, such as oil or the like. A control arrangement is provided which operates in synchronism with the operation of the internal combustion (IC) engine to effect connection between a pressure source for the servo fluid and the servo pressure area, by selectively controlling a valve, typically a spool valve which has a slider spool. The quantity of fuel to be injected is determined by placing the pump working space or chamber in pressure-free condition before the pump piston actually carries out its pressure stroke. A certain quantity of fuel, predetermined in accordance with operating parameters of the engine, is then injected under substantial pressure as soon as the pressurized servo fluid is admitted to the servo cylinder to operate the servo piston, and hence the pressure piston which actually injects the fuel. In this system, the servo liquid is controlled by a spool valve and so conducted into a servo cylinder or servo pressure chamber that the servo piston then can control, directly, the cross section for the servo fluid duct. This arrangement permits influencing the injection mode, but does not provide for adequate control thereof and thus the servo-injection is difficult to apply widely. To control the servo fluid, typically an oil, the control slider has control edges which, by fitting against ports in the spool housing for the control slider, respectively control supply and drain of the servo liquid.
It has been found that the shape of the rising flank of the injection pulse of the fluid, upon injection, should be accurately controlled; this is difficult, if not impossible to achieve, with known structures.